1. Field of the Invention
The present invention relates to a capacitor microphone including a polarization voltage generating circuit, and more particularly to a capacitor microphone including a polarization voltage generating circuit in which polarization voltage can be controlled.
2. Description of the Related Art
Capacitor microphones generally include a capacitor microphone unit mainly including a diaphragm that vibrates upon receiving sound waves and a fixed electrode arranged to be opposite to the diaphragm. A peripheral portion of the diaphragm is attached to a diaphragm holding ring with appropriate extension force applied to the diaphragm. A ring-shaped spacer is provided between the fixed electrode and a portion of the diaphragm attached to the diaphragm holding ring, thereby providing a minute space defined by the thickness of the spacer between the fixed electrode and the diaphragm. The diaphragm and the fixed electrode are incorporated in a unit casing of the microphone unit along with other elements such as an insulating base, a field effect transistor (FET) serving as an impedance converter, and a circuit board. The elements are positioned and fixed in the unit casing.
The diaphragm is connected to, for example, the anode side of a power source for polarization via the diaphragm holding ring and the unit casing. The fixed electrode is connected to, for example, the cathode side of the power source for polarization via a leading terminal penetrating the insulating base to protrude therefrom. Thus, polarization voltage is applied across the diaphragm and the fixed electrode, whereby a capacitor is formed by the diaphragm and the fixed electrode.
When the diaphragm vibrates upon receiving sound waves, the distance between the diaphragm and the fixed electrode changes and the capacity of the capacitor changes. This is output as a change in current across the diaphragm and the fixed electrode. Impedance of such an output is extremely high. Thus, an output from the impedance converter formed by the FET, which lowers the impedance, serves as an output from the microphone unit.
The sensitivity of the capacitor microphone having the structure described above depends on the polarization voltage applied across the diaphragm and the fixed electrode. Higher polarization voltage is directly related to higher sensitivity. Therefore, polarization voltage is generated by increasing DC power voltage by a DC boost circuit, i.e., a DC-DC converter. Unfortunately, due to other circuits in the microphone, the DC-DC converter is only capable of receiving current of 1 mA or lower. Thus, the polarization voltage cannot be increased over a certain level. To further increase the polarization voltage, the DC-DC converter is provided with a rectifying circuit of a voltage multiplier configuration. FIG. 5 exemplarily illustrates a polarization voltage generating circuit, included in a conventional capacitor microphone, in which a rectifying circuit of the DC-DC converter has a voltage multiplier configuration.
The polarization voltage generating circuit illustrated in FIG. 5 is configured as a DC-DC converter that has a power input terminal 10 connecting to a phantom power source. The polarization voltage generating circuit mainly includes an oscillating circuit 12 and a rectifying circuit 14 of a voltage multiplier configuration. The oscillating circuit 12 is mainly composed of a transistor TR. A variable resistor VR is connected between the power input terminal 10 and the collector of the transistor TR. A capacitor C5 is connected between the power input terminal 10 and the emitter of the transistor TR. A resistor R is connected between the collector and the base of the transistor TR. A coil L2 is connected between the emitter of the transistor TR and earth. The base of the transistor TR is connected to one end of a capacitor C4 and the emitter of the transistor TR is connected to one end of a coil L1. The other ends of the capacitor C4 and the coil L1 are connected with each other. Thus, the capacitor C4 and the coil L1 are connected in series between the base and the emitter of the transistor TR.
The coils L1 and L2 are electromagnetically inductively coupled by, for example, being wound around a common core. The oscillating circuit 12 alternately oscillates. The coils L1 and L2 are electromagnetically inductively coupled. The turn ratio between the coils L1 and L2 is approximately 1 to 8. Thus, DC voltage of about 5 V input through the power input terminal 10 is boosted to AC voltage of about 40 V. This AC voltage is converted into high DC voltage by the rectifying circuit 14 of a voltage multiplier configuration described below. The current flowing through the oscillating circuit 12 can be adjusted by the variable resistor VR to obtain appropriate polarization voltage as described below.
The rectifying circuit 14 of a voltage multiplier configuration includes four diodes D1 to D4 and three capacitors C1 to C3. The four diodes are connected in series in the forward direction between the earth and an output terminal 16 in order of D4 to D1 from the earth to the output terminal 16. Thus, the anode of the diode D4 is connected to the earth while the cathode of the diode D1 is connected to the output terminal 16. The capacitor C1 is connected between a connection point of the diodes D2 and D3 and the output terminal 16. The capacitor C2 is connected between connection points of the diodes D1 and D2 and the diodes D3 and D4. The capacitor C3 is connected between a connection portion of the diodes D3 and D4 and a connection portion of the capacitor C4 and the coil L1. Thus, the rectifying circuit 14 has a voltage tripler configuration. Therefore, the AC voltage of about 40 V from the oscillating circuit 12 is boosted to DC voltage of about 100 to 120 V. The DC voltage obtained by the boosting is output from the output terminal 16 as polarization voltage of the capacitor microphone directly or after being smoothed by a smoothing circuit (not illustrated).
Japanese Patent Application Publication H9-121533 discloses a polarization voltage generating circuit similar to that illustrated in FIG. 5.
A conventional capacitor microphone having the polarization voltage generating circuit as illustrated in FIG. 5 obtains appropriate polarization voltage by adjusting current flowing through the transistor TR for oscillation using the variable resistor VR, as described above. Operating condition of the oscillating circuit 12 fluctuates because the current flowing through the transistor TR is variable. Therefore, the current that can be supplied to the polarization voltage generating current cannot be specified or is limited. Thus, individual difference in consumption current of the microphone may be produced and the output voltage from the output terminal 16 may fluctuate. The same problem occurs when other conditions such as coupling level of the coils L1 and L2 change.